The disclosure of Japanese Patent Application No. 2000-118877 filed on Apr. 20, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a tire air pressure determining apparatus that determines whether or not a tire air pressure of a vehicle is normal and, more particularly, to a tire air pressure determining apparatus that does not erroneously determine, when a tire air pressure is actually normal, that the tire air pressure is not normal, even in the case where wheels are fitted with tires that are different in dependency of dynamic load radius on vehicle speed.
2. Description of Related Art
An apparatus of this type, e.g., as disclosed in Japanese Patent Application Laid-Open No. 9-286213, has wheel speed sensors that generate pulses in accordance with rotation of wheels, and is designed to calculate a difference xcex2 between a sum (PFR+PRL) of a wheel speed pulse number (cumulative value) PFR of a front-right wheel and a wheel speed pulse number (cumulative value) PRL of a rear-left wheel within a predetermined period and a sum (PFL+PRR) of a wheel speed pulse number (cumulative value) PFL of a front-left wheel and a wheel speed pulse number (cumulative value) PRR of a rear-right wheel within the predetermined period (the dynamic load radial deviation xcex2=(PFR+PRL)xe2x88x92(PFL+PRR)), determine, if the difference xcex2 is greater than a predetermined value, that an air pressure of one of the wheels is not normal, and advise a passenger of a result of determination by means of an alarm such as a lamp, etc.
However, the vehicle may be fitted with two types of tires, i.e., tires having a dynamic load radius that greatly changes in accordance with a change in vehicle speed and thus depending greatly on vehicle speed, and tires having a dynamic load radius that slightly changes in accordance with a change in vehicle speed and thus depending slightly on vehicle speed. In the above-mentioned apparatus, in the case where the vehicle is fitted with both types of tires, the absolute value of the dynamic load radial deviation (referred to also as a dynamic load radius relative comparison value) xcex2 may become great despite the fact that tire air pressures of all the wheels are normal. This causes a problem of erroneously determining that a tire air pressure is not normal.
It is an object of the invention to provide a tire air pressure determining apparatus that does not erroneously determine that a tire air pressure is not normal, even in the case where a vehicle is fitted with both types of tires, i.e., a tire that is greatly dependent on vehicle speed and a tire that is slightly dependent on vehicle speed.
The invention has been made to achieve the above object. A tire air pressure determining apparatus of the invention has wheel speed pulse generating means that generates a pulse every time wheels of a vehicle rotate by a predetermined angle and that is provided for each of the wheels, and determines, based on a pulse generated by the wheel speed pulse generating means, whether or not a tire air pressure is normal. This tire air pressure determining apparatus comprises vehicle speed detecting means that detects a speed of the vehicle, variable value calculating means that calculates a variable value for making determination of a tire air pressure based on a pulse generated by the wheel speed pulse generating means when a predetermined condition is fulfilled, reference variable value storing means that stores the calculated variable value as a reference variable value, reference vehicle speed storing means that stores a speed of the vehicle at the time when the reference variable value is calculated as a reference vehicle speed associated with the reference variable value, vehicle speed condition determining means that determines whether or not a vehicle speed at the time when the variable value calculating means newly calculates the variable value is within a predetermined vehicle speed range determined by the reference vehicle speed, and air pressure determining means that determines, based on a difference between the newly calculated variable value and the reference variable value associated with the reference vehicle speed, whether or not the tire air pressure is normal, if it is determined that the vehicle speed at the time when the variable value calculating means newly calculates the variable value is within the predetermined vehicle speed range determined by the reference vehicle speed.
According to the above-mentioned construction, a variable value for making determination of a tire air pressure is calculated based on a pulse generated by the wheel speed pulse generating means provided for each of the wheels upon fulfillment of the predetermined condition, and the calculated variable value is stored as a reference variable value, and a speed of the vehicle at the time when the reference variable value is calculated is stored as a reference vehicle speed associated with the reference variable value. If the variable value is newly calculated, it is determined whether or not a vehicle speed at that moment is within a predetermined vehicle speed range determined by the reference vehicle speed. If it is determined that the vehicle speed is within the predetermined vehicle speed range, it is determined, based on a difference between the newly calculated variable value and the reference variable value associated with the reference vehicle speed, whether or not a tire air pressure is normal.
That is, in the above-mentioned construction, if it is determined that the vehicle speed at the time when the variable value is newly calculated is within the predetermined vehicle speed range determined by the reference vehicle speed, it is determined, based on a difference between the newly calculated variable value and the reference variable value associated with the reference vehicle speed, whether or not the tire air pressure is normal. Therefore, in the case where the variable value for making determination of a tire air pressure is free from the influence of dependency of dynamic load radius on vehicle speed (strictly speaking, the variable value is susceptible to the influence only to a permissible extent), determination of a tire air pressure is made based on the variable value. Thus, even if the vehicle is fitted with tires that are different in dependency of dynamic load radius on vehicle speed, the possibility of erroneously determining that a tire air pressure is not normal is eliminated.